The present invention is in the field of drive circuits for solenoids, particularly for use as a brake release in connection with a motor.
A prior art reversible motor 10 is shown in FIG. 1. The motor is a three terminal capacitor-start capacitor-run motor, including two wound coils. One terminal of the motor is connected to a clockwise winding. A second terminal is connected to a counterclockwise winding in the motor. A third terminal is for connection to the AC common input. A capacitor 12, referred to herein as the motor capacitor, is connected between the first and second windings to impose a phase angle shift on the AC input voltage to the opposite coil. The motor capacitor is selected to nominally provide about a 90xc2x0 phase shift. An AC input voltage may be applied either to the clockwise winding or the counterclockwise winding to cause the motor to turn in one direction or the other. It has been found desirable in such a motor to provide a brake to more rapidly stop the motor when voltage is removed from the inputs. A typical brake is spring loaded so as to be biased into the stop position. A solenoid 14 is used to retract the brake when power is applied to the motor. The solenoid 14 is connected between the first and second terminals. When power is applied to either the first or second terminals of the motor, the solenoid is energized retracting the brake. The problem with this design is that the voltage across the two terminals and thus across the solenoid, is approximately twice the magnitude of the AC input voltage. For a 115 volt line, a solenoid is required that can handle over 200 volts. For a motor that plugs into a 230 volt line, the solenoid would need to be able to handle over 400 volts. Such solenoids are not commonly found and are unnecessarily expensive.
In accordance with an embodiment of the invention, a solenoid drive circuit is incorporated into the circuitry accompanying the motor in order to reduce the voltage across the solenoid. A capacitor is provided in series with an AC solenoid so as to divide the AC voltage between two terminals. Thus, an advantageously reduced voltage is provided across the solenoid to energize it for releasing the motor brake.
In an alternative embodiment of the invention, the brake release is formed by a DC solenoid. A diode bridge rectifier circuit is connected across the DC solenoid to steer an AC current through the DC solenoid in only one direction. A capacitor is coupled in series with the diode bridge rectifier and solenoid to divide the AC voltage. Thus, the DC voltage produced across the solenoid is reduced in magnitude relative to the AC voltage across the terminals. Thus, a reversible motor with an AC input voltage may be supplied with the brake release advantageously operated by a common low voltage solenoid.